At least a fivefold gap in effective global marine protection.

The Convention on Biological Diversity's ambitious target to "effectively conserve" 30% of the world's oceans in protected areas by 2030 risks being undermined by a primary focus on total area protected rather than realized conservation outcomes. Area-based assessments can overstate success because few marine protected areas (MPAs) possess strong restrictions and high compliance. Here, we estimate the extent that conservation outcomes are realized across the global MPA network for shallow reefs using standardized fish survey data. We first compare observed fish biomass from 552 surveyed MPA zones in temperate and tropical seas to modeled counterfactuals in the absence of protection, then extend these findings across the global network using key MPA attributes and predictive models. We estimate that at best, a quarter of the global MPA network is likely delivering meaningful conservation outcomes, with effectiveness strongly contingent upon the level of compliance with fishing restrictions. If shallow reef responses are representative of other marine ecosystems, then <2% of the ocean is effectively protected, compared to 9.8% presently reported. The unsupported assumption that MPA designation and management attributes directly translates to tangible biodiversity outcomes has apparently resulted in pervasive overestimation of conservation achievement. Critically, long-term success of MPAs will require further research, informed governance, and improved strategic policies that focus on the intricate mechanisms underlying compliance with mandated area restrictions.